1. Field of the Invention
The invention relates to an integrated micromechanical sensor device and to a process for its production.
Micromechanical sensors are becoming increasingly established in all fields of technology, for example in navigation systems and motor vehicles, in particular in conjunction with safety systems. A large proportion of such sensors form pressure and acceleration sensors. There is a requirement for reliable and compact sensors which are easy to produce and therefore inexpensive, with high measurement accuracy and good proportionality between the measured quantity and the output signal.
2. Description of the Related Art
Most pressure or acceleration sensors currently used are produced using high-precision mechanical methods or by using KOH etching technology on a silicon base (bulk micromachining). The sensor signal, to date usually produced by the piezoelectric effect, is evaluated separately from the sensor. There is, however, a trend toward intelligent sensors in which the sensor as well as the circuit for evaluating the sensor signal, and optionally a test circuit, are integrated on a chip on the basis of planar silicon technology. Evaluation of the piezoresistive or capacitive sensor signal, and linearization and amplification, take place using semiconductor circuits of known technologies. Such a sensor is, for example, disclosed by the published article F. Goodenough: Airbags Boom When IC Accelerometer Sees 50 G, Electronic Design, Aug. 8, 1991, pp. 45-56.
Whereas conventionally produced micromechanical sensors are relatively large, expensive and inaccurate, the abovementioned published article describes an improved embodiment. The production of this known so-called surface micromechanical (surface micromachining) sensor requires, as emerges in particular from the related further published article: Analog Devices Combine Micromachining and BICMOS, Semiconductor International, Oct. 1991, 21 masks, namely 6 masks for the sensor process and 15 masks for a 4-.mu.m BICMOS process. The comb-shaped sensor element for forming the capacitive sensor consists of a 2 .mu.m thick polysilicon element and is connected to the substrate surface via springs, which are also made of polysilicon.
The production process for the known sensor is extraordinarily elaborate and expensive. Moreover, it is not certain that the polysilicon layers used for the mechanically moved parts of the sensor have sufficient long-term mechanical stability. In addition to this possible degradation over time, the mechanical properties such as the elastic modulus or intrinsic stress of polysilicon is sensitively dependent on the respective process conditions during production. The thermal annealing of the intrinsic stress requires additional heat-treatment steps in the production process, which has a detrimental effect on the electronic circuit simultaneously integrated in the sensor. Further to this, additional depositions of semiconductor layers are required in the production process. In one conceivable use of modern sub-.mu.m-BICMOS circuits for the evaluation circuit of the sensor, the low process temperatures then used make it no longer possible to produce stress-free polysilicon layers.
DE-A-43 09 917, published after the priority date of the present application, describes the use of a monocrystalline silicon layer with an overlying silicon nitride layer.